//生命周期


fn main() {
    let x: &Vec<i32>;
    {
        let y = Vec::new(); //----+             //                               | y's lifetime             //                               |
        x = &y; //----------------|--------------+      //                               |              |
    } // <------------------------+              | x's lifetime
    println!("x's length is {}", x.len()); //    |
}
